client: Replace `unsafe_rpc_expose` with an `RpcMethods` enum (#5729)

* client: Replace `unsafe_rpc_expose` with an `RpcMethods` enum

which can be either Default, Safe or Unsafe. The idea is to have the
following:
|                       | --rpc-external=false  | --rpc-external=true   |
|---------------------  |-------------------    |-----------------      |
| --rpc-methods=Default |                       | unsafe calls denied   |
| --rpc-methods=Safe    | unsafe calls denied   | unsafe calls denied   |
| --rpc-methods=Unsafe  |                       |                       |
Since the previous `unsafe-rpc-expose` option was confusing.

* client: Only warn against exposing externally unsafe RPC method set

* Apply suggestions from code review

Co-Authored-By: Cecile Tonglet <cecile.tonglet@cecton.com>

* cli: Rephrase doc comment for rpc_methods config

* Improve debuggability of build_spec_works

...by printing to stderr the stderr of the command. This is normally
suppressed for succesful tests but not for failing ones - if that's the
case then it's useful to see the test failure reason inline rather than
having to execute the command separately ourselves.

* Rename RpcMethods::{Default => Auto} variant

* Update bin/node/cli/tests/build_spec_works.rs

Co-authored-by: Benjamin Kampmann <ben.kampmann@googlemail.com>
Co-authored-by: Cecile Tonglet <cecile.tonglet@cecton.com>
Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>